1. Field of the Invention
The present invention relates to a rotating anode for an X-ray tube, simultaneously emitting the radiation spectra of two distinct materials.
2. Description of the Prior Art
An X-ray tube is constituted by a rotationally symmetrical block, for example a disk or truncated cone, with a substrate that is coated on one face with a layer of material having a high atomic number, namely a metal such as molybdenum, tungsten or rhodium for example. In a glass bulb in which a high vacuum prevails, there is placed a cathode made of a thermoelectronic emitter such as a tungsten filament, designed to emit an electron beam focused on the anode which is itself enclosed in the bulb. When the filament of the cathode is made incandescent and when the anode has been taken to a positive potential of some kilowatts with respect to the cathode, then the electrons emitted by the cathode are accelerated towards the anode by the electrical field created between the two electrodes, and they impinge upon a surface of the anode called the X-ray focal spot. This zone of impact of the electrons becomes the main source of emission of the X-rays throughout the space facing the anode, except at the glancing incidences. In another type of embodiment, it is the edge of the anode that is coated with a layer of material having a high atomic number, the cathode being then positioned in such a way that it emits an electron beam on the focal spot of the anode located on the edge.
The radiation output of an X-ray tube depends on factors such as the stream of electrons, the difference in potential between the cathode and the anode and the atomic number of the material constituting the target of the electron beam.
In the application of an X-ray tube to medical diagnosis, it is important that the properties of this tube should enable very high quality of the exposures obtained, taking account of the tissues to be radiographed. The quality of the exposures is defined by the sharpness and the contrast which are related to the following factors:
--the distribution in density of the electrons, emitted by the cathode, on the surface of the focal spot of the anode; PA1 --the place at which the secondary electrons emitted by the cathode or any other parasitic emission fall, outside the focal spot of the anode; PA1 --the spectrum of the X-radiation used in relation to the object to be radiographed.